Indium phosphide (InP) substrates, on account of features including that they possess light-emitting characteristics and that they have a high electron drift velocity, are widely employed in components including semiconductor lasers, LEDs (light-emitting diodes), and high-speed microelectronic devices. With semiconductor lasers and LEDs, by measuring the PL intensity after the epitaxial layers have been formed onto the InP substrate, an assessment of the light-emitting performance can be simply and readily made.
The PL light-emitting intensity should be strong. And with high-speed devices, since impurities at the interface between the InP substrate and the epitaxial layers lead to leakage problems, the characteristics stabilize the fewer are n-type or p-type impurities at the interface.
An example of a method of manufacturing InP substrates is disclosed in Japanese Unexamined Pat. App. Pub. No. H05-267185 (Patent Reference 1). Specifically, an ingot is cut into wafers and then the wafers are polished to a specular finish. The front side of the optically flat wafers, having chemical components of the polishing agents remaining on it, becoming oxidized after the polishing, or having organic matter clinging to it, will have formed thereon a layer (front-side film) that differs from the ingot. Etching removal of the front-side film, employing sulfuric acid, is carried out to leave the thickness of the front-side film not greater than 1 nm.
Examples, furthermore, of InP substrate front-side treatments employing solutions other than sulfuric acid are disclosed in Japanese Unexamined Pat. App. Pub. No. H05-166785 (Patent Reference 2), Japanese Unexamined Pat. App. Pub. No. S54-13500 (Patent Reference 3) and Japanese Unexamined Pat. App. Pub. No. S62-252140 (Patent Reference 4). Patent Reference 2 discloses washing InP wafers in a substance blend of phosphoric acid-hydrogen peroxide-water, or in a liquid blend of hydrogen fluoride-hydrogen peroxide-water. Patent Reference 3 discloses etching InP single-crystal wafers with sulfuric acid in which the hydrogen peroxide is at or under 15 by volume. Patent Reference 4 discloses washing InP wafers by means a solution blend of phosphoric acid-hydrogen peroxide-water, or a solution blend of hydrogen fluoride-hydrogen peroxide-water.